1. Field of the Invention
The present invention relates to a high-speed photodiode, especially relates to a homojunction type one, in particular, to a homojunction type high-speed photodiode which has a graded bandgap absorption layer with huge optical absorption constant (>2 μm−1) to absorb more than 95% of incident photons to avoid any surface recombination. Furthermore, the homojunction type high-speed photodiode has an epitaxial layer structure of indium gallium arsenide (InGaAs) which avoids any absorbance while offers the effect of ultra-high-speed electron drift.
2. Description of Related Art
The light emitting diode is able to convert optical signals into electrical signals of components. Therefore, the response speed and quantum efficiency are essential for a high-speed photodiode. For optical interconnect (OI) applications, high-density VCSEL arrays requires easy coupling of light and the light detector array with high density. Therefore, an optical detector of normal incidence structure is much more suitable than other structures. However, the optical detector usually requires external reverse bias (−3 to −5V) to maintain the high-speed operation, resulting in the electric power consumption and the demand for additional bias circuit.
A traditional PIN light diode consists of a depletion layer (i.e., I area) of narrow bandgap located between a P-type layer of wide bandgap and an N-type layer. The absorbance area locates in the depletion layer. Incident photons are absorbed in the depletion layer. At the same time, electron-hole pairs are stimulated to form. The electrons and electrical holes are swept into the P-type layer and the N-type layer due to acceleration by an electric field in the depletion layer, and form a photocurrent accordingly. Taking into account that the hole speed is much slower than the electron speed, the hole tends to accumulate in intrinsic area, causing the electric field shielding effect which makes the internal electric field smaller. In such situation, carriers discharge slows down, adversely affecting the output power. The increase in the thickness of the depletion layer contributes to the decrease in the limit to the RC bandwidth. If the depletion layer too thick, it results in the carrier drift time too long and the response speed becomes slow. If the thickness of the depletion layer reduces, then the carrier drift time can be shortened while the saturation current increases. Whereas, it will cause the capacitance to become larger, the bandwidth to decrease, and the quantum efficiency to decrease accordingly. In addition, since the equivalent mass of the holes is heavy, such components must usually be operated at bias of more than −3 volts (V) in order to accelerate the transfer of the holes. Thus, it is obvious that the component speed will become too slow due to the electric field inside the depletion layer is too small and the hole drift speed is too slow, if the traditional PIN light diode is operated with any bias. To increase the built-in electric field needs to enlarge the energy gap of depletion layer. However, such an approach will result in a substantial decline in the efficiency of the absorbance.
A single-carrier transmission light emitting diode usually has a P-type narrow band absorption layer and a wide band collector layer. The P-type narrow band absorption layer is usually neutral and most of its carrier (hole) can quickly flip the sheets to the metal it contacts. Therefore, in a single-carrier transmission optical detector, the electron can be said to be the sole working charge. The electron transmission time (including the time the electrons passing through the transport layer and the absorbing layer) determines the transient time of the single-carrier transmission light detector. Currently, the InGaAs—InP single-carrier transmission light emitting diode has been widely used at the optical communication band of 1550 nm. If it is used at the optical communication band of 650 nm, the energy absorbed by the light emitting diode made of InP is fairly large, disadvantageously collecting unwanted electron-hole pairs in the collection layer. The special electric field effect caused by the holes staying in the absorbance area will reduce the high-speed performance of the components.
Recently, with the use of the single-carrier transmission light detector structure, the zero-bias operation demonstrates the performance of fast and appropriate response. Thus, for fulfilling the demand in the green Internet in recent years, the application of the light emitting diode in the optical link sector have attracted more and more attention. However, the current technology cannot satisfies the requirement that the diameter of the active area should be greater than 50 microns (μm) in order to facilitate optical alignment and package while retaining the operation at high speed (25 Gbps) and low energy consumption of component. Therefore, there is a need of a new structure that can meet the need for the user in practical use.